For the Crust
Preheat oven to 350°F.
Using a food processor or mini-chopper, pulse the graham crackers until finely ground. In a medium bowl, combine the graham cracker crumbs, brown sugar and melted butter. Stir until fully combined.
Press mixture into bottom of 10” cast iron skillet. (Tip: A flat-bottomed glass helps pack the crust mixture down evenly.)
Bake crust at 350°F for 8 minutes.
Remove skillet from oven and set aside to cool.
For the Brownie Filling
Using a medium saucepan, add the butter, unsweetened chocolate and 1 cup of chocolate chips. Place over medium-low heat, stirring often, until butter and chocolate have fully melted; set pan aside to cool for at least 10 minutes.
In a large bowl, mix together the eggs, sugar and vanilla. Stir the egg mixture into the warm chocolate mixture.
In a medium bowl, sift together the flour, baking powder and salt. Stir the flour mixture into the chocolate mixture.
Stir 1½ cups of marshmallows and the remaining ¾ cup of chocolate chips into the batter. Pour batter on top of graham cracker base. Sprinkle remaining 1 cup of marshmallow on top.
Bake at 350°F for 80-85 minutes, or until a toothpick inserted into the filling comes out mostly clean.
Let cool before serving.